Calcitonin is a naturally occurring hormone with a short half-life that is believed to act directly on osteoclasts (via receptors on the cell surface for calcitonin). This action may directly inhibit osteoclastic bone resorption, which may lead to hypocalcemic and/or hypophosphatemic serum effects. Calcitonin may be useful in treating various bone disorders including, but not limited to, osteoporosis and Paget's disease.
Osteoporosis is a bone disease in which bone tissue is normally mineralized, but the amount of bone is decreased and the structural integrity of trabecular bone is impaired. Cortical bone becomes more porous and thinner. This makes the bone weaker and more likely to fracture. In the United States, about 21% of postmenopausal women have osteoporosis (low bone density), and about 16% have had a fracture. In women older than 80, about 40% have experienced a fracture of the hip, vertebra, arm, or pelvis. The population of older men and women has been increasing, and therefore the number of people with osteoporosis is increasing.
Calcitonin given as a subcutaneous injection has shown significant improvements in bone density; however, a high incidence of side effects, including pain at the injection site, flushing and nausea, have been reported which may limit the use of the drug.
Paget's disease of bone is a metabolic bone disorder of unknown origin which normally affects older people. The disease causes an increased and irregular formation of bone as the bone cells, which are responsible for dissolving the body's old bone and replacing it with new, become out of control. Over a period of time the deformed new bone becomes larger, weaker and has more blood vessels than normal bone. Unlike normal bone, the structure is irregular and consequently weaker, which makes it prone to fracture even after a minor injury.
In its mildest form the disease has no symptoms. In more severe cases the pain can be intense. The relentless progression of the disease may cause bones to bow, the skull may increase in size and the spinal column may curve. As the bones enlarge they may cause pressure on nearby nerves which can result in muscle weakness. In the case of severe skull enlargement this pressure can result in deafness, disturbed vision, dizziness and tinnitus.
Calcitonin may be effective in treating disorders of increased skeletal remodeling, such as Paget's disease. In treating Paget's disease, chronic use of calcitonin may produce long-term reduction in symptoms; however, side effects of calcitonin administration may include nausea, hand swelling, urticaria, and intestinal cramping.
Various references have proposed conjugating polypeptides such as calcitonin with polydispersed mixtures of polyethylene glycol or polyethylene glycol-containing polymers. For example, U.S. Pat. No. 5,359,030 to Ekwuribe proposes conjugating polypeptides such as calcitonin with polydispersed mixtures of polyethylene glycol modified glycolipid polymers and polydispersed mixtures of polyethylene glycol modified fatty acid polymers. The number average molecular weight of polymer resulting from each combination is preferred to be in the range of from about 500 to about 10,000 Daltons.
The polydispersity of the polymer mixtures and conjugates described in Ekwuribe is likely a result of the use of polydispersed polyethylene glycol in the polymer synthesis. PEG is typically produced by base-catalyzed ring-opening polymerization of ethylene oxide. The reaction is initiated by adding ethylene oxide to ethylene glycol, with potassium hydroxide as catalyst. This process results in a polydispersed mixture of polyethylene glycol polymers having a number average molecular weight within a given range of molecular weights. For example, PEG products offered by Sigma-Aldrich of Milwaukee, Wis. are provided in polydispersed mixtures such as PEG 400 (Mn 380–420); PEG 1,000 (Mn 950–1,050); PEG 1,500 (Mn 1,400–1,600); and PEG 2,000 (Mn 1,900–2,200).
It is desirable to provide non-polydispersed mixtures of calcitonin drug-oligomer conjugates where the oligomer comprises polyethylene glycol.